OCD and intrusive thoughts

Can anyone explain ocd to me and how you can tell if intrusive thoughts are caused by ocd.

I think it was poltergoose who recommended the book What to do when your brain gets stuck. I am working my way through it with ds and it does seem to be helping.

DS is only 6 so too young to be diagnosed. I am wondering how a doctor diagnoses ocd. Would DS have to explain his thoughts? This weekend he has been crying or shouting "mummy help me get this thought out of my head, I can't get it out". Would you say this sounds like ocd? In the past he has had the rituals but has not been able to communicate anything about his thoughts until now.